The demand for networked digital audiovisual systems is expected to grow considerably over the next few years, as businesses, government and other institutions increasingly turn to digital networks to distribute live and/or pre-recorded audiovisual information for education, presentations and reference applications. These customers expect systems that will allow a number of people to be able to view audiovisual information simultaneously, while fully retaining their other network functions. For example, in business computing, most of the major productivity software developers see networked live or stored video as an effective means of training and supporting users. Many of these developers have begun including VHS videotapes for training with their software. Now they want to take this a step further by linking live or stored video directly to their software's on-line help resources. Centralizing that support reduces the cost for customers with many users and ensures that it is properly maintained by the MIS department or other responsible.
Networked video presentation systems in business can allow corporate resources, such as sales videos, employee information, and live video-based training to be available immediately to all employees from their desks. Similarly, networked video documentation systems will allow institutions of all kinds to maintain multi-user audiovisual databases. The largest population of users of such systems are likely to be health care institutions which have extensive audiovisual records. Such databases can be used for on-the-job reference such as revisiting a complex procedure on the manufacturing floor, or creating on-line archives of TV commercials for an advertising agency.
Video teleconferencing is a fast growing segment in the communications arena. However, most of today's live video teleconferencing installations are dedicated, standalone facilities set aside for that purpose. While the price tag for such systems is dropping, it remains an expensive facility that needs to be shared. Extending these services to the desktop over local area networks will make live video teleconferencing services more widely available at a lower cost.
Finally, the ultimate goal is computer-supported collaboration, whereby users at different locations will be able to share stored and live video data and work on problems simultaneously using multimedia workstations linked by local and wide-area networks. Users of such desktop conferencing systems will be able to access stored video and audio from a central server, hold live audio/video conferences with remotely based colleagues via the PCs on their desks and work simultaneously with them on files in a shared electronic workspace.
These examples clearly show that the support of digital video services preferably includes the communication of live full motion video information over digital networks.
The characteristics of files, file access and network traffic in digital video applications differ substantially from those encountered in data applications.
With data applications, whenever a user makes a file access request to a server, or requests that data be transmitted on a network, the user expects a fast response time relative to the time it takes it to place the next request. As a result, the capacity of a server and the overall network bandwidth must both be large compared to the average demand placed by a single user. Accordingly, the design of a file server aimed at supporting data applications and the design of a network to support data traffic have been based on the principle of bandwidth sharing and statistical time multiplexing. For example, local area networks of the Ethernet type (10 Mbits/s) and of the token-ring type (4 and 16 Mbits/s) serving tens to hundreds of users have proliferated. File servers have furthermore taken advantage of the property of locality in file access, and incorporated appropriate caching mechanisms. In all cases, as the overall load placed on the shared resources increased, the average response time experienced by all users also increased.
Let us examine now digital video. A video signal is analog in nature and continuous over time. It is digitized by first sampling it at regular intervals, and then by quantizing each sample. This digitization process results in a data stream which is of relatively constant and very high rate; (NTSC signals result in data rates in the neighborhood of 100 Mb/s, and an HDTV signal, 600 Mb/s.) However, given that the sampled data exhibits a great deal of redundancy, compression is applied, thus significantly reducing the stream's rate. Depending on the bandwidth of the original analog signal, the sampling rate, the quantization step size, the encoding method, and the desired image quality, the resulting data rate for a digital video signal can range from 64 Kb/s to tens of Mb/s. For example, CCITT Recommendation H.261 specifies video coding and decoding methods for audio visual services at the rate of px64 Kbits/s, where p is in the range of 1 to 30 (i.e., 64 Kb/s to 2 Mb/s); the MPEG standard specifies a coded representation that can be used for compressing video sequences to bit rates around 1.5 Mbits/s, and its successor, known as MPEG II, provides a wider range of functionality and image quality at rates in the range of 4 to 8 Mbits/s. Advances in compression techniques and in their VLSI circuit implementations are among the important reasons why video services over LANs are becoming practical.
Two important observations may be made. The first is that the volume of bits corresponding to a digitized video segment of useful duration (even compressed) is large. A ten minute DVI video segment requires 90 Mbytes of storage; ten hours require over 5 Gbytes. Thus video servers where shared video information is to be stored must have relatively large storage capacity.
The second observation is that the communication of digital video data between two nodes on a network (e.g., a stored video stream transmitted between a server and a desktop station, or a live video stream transmitted between two desktop stations) requires that data be transmitted in a stream fashion. This means that data packets must be delivered to the destination on time, and failure to deliver data on time would result in video quality degradation. (This characteristic has earned this type of traffic the attribute synchronous or isochronous.) This has two main implications: (i) from a network's point of view, one requires the availability, on a continuous basis, of a bandwidth at least equal to the signal's data rate; (note that the data rate associated with a digitized video signal, even compressed, is larger than the average traffic rate for a typical data application user;) (ii) from a file and storage system point of view, one requires streaming capabilities which guarantee the continuity of each stream being retrieved or stored. Thus, in order to support multiple independent video signals, the network must have the necessary aggregate bandwidth as well as means to guarantee the bandwidth required for each video signal, and the file and storage system must be of the streaming type and must have a capacity sufficient to handle all video streams. By the same token, there is a maximum number of video streams of a given data rate that a network and a server can support, and means must exist to prevent additional requests from overloading the system. While in data applications an overload results in higher response time, with live video applications, any additional load beyond the maximum possible would result in degraded video quality.
It is thus clear that the characteristics of video traffic differ substantially from those of traditional data traffic to the point that servers and local area networks designed primarily to support data applications are not appropriate to effectively support video services. New capabilities in servers and networks must be offered.
Accordingly, it is an object of the present invention to provide a video server suitable for supporting a wide variety of live full motion video applications as well as integrating live video distribution with other functions including video recording and playback.